configure: when compiling with CLANG, set -std=gnu89

[Austin English]

Clang defaults to compiling in C99 mode, which breaks the compile.

-- 
-Austin
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diff --git a/configure b/configure
index 6db971a..04595a8 100755
--- a/configure
+++ b/configure
@@ -4008,6 +4008,12 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+case $CC in
+  clang)
+     CFLAGS="$CFLAGS -std=gnu89"
+     ;;
+esac
+
 case $host in
   *-darwin*)
     if test "x$enable_win64" = "xyes"
diff --git a/configure.ac b/configure.ac
index bd1276b..9c16124 100644
--- a/configure.ac
+++ b/configure.ac
@@ -116,6 +116,13 @@ dnl We can't use AC_PROG_CPP for winegcc, it uses by default $(CC) -E
 AC_CHECK_TOOL(CPPBIN,cpp,cpp)
 AC_DEFINE_UNQUOTED(EXEEXT,["$ac_exeext"],[Define to the file extension for executables.])
 
+dnl Clang defaults to gnu99 mode, while Wine needs gnu89
+case $CC in
+  clang)
+     CFLAGS="$CFLAGS -std=gnu89"
+     ;;
+esac
+
 case $host in
   *-darwin*)
     if test "x$enable_win64" = "xyes"